Wireless communication techniques based on multiple subcarriers, such as an orthogonal frequency-division multiplexing (OFDM) technique, are gaining worldwide popularity due to their broad applications. For example, an OFDM based communication system may be used in a plurality of networks including Worldwide Interoperability for Microwave Access (WiMAX) networks, Wireless Fidelity (Wi-Fi) networks, Wireless Broadband (WiBro) networks, etc.
A transmitter in an OFDM based communication system may use a plurality of closely-spaced orthogonal subcarriers to carry data. For example, the transmitter may allocate the data on a plurality of parallel data channels, one for each of the subcarriers. Each of the subcarriers may be modulated with a conventional modulation scheme, e.g., quadrature amplitude modulation, at a relatively low symbol rate. In addition, the transmitter may perform an inverse fast Fourier transform (IFFT) on OFDM symbols representing the data to be transmitted, and transmit signals including the OFDM symbols to a receiver in the communication system. The receiver may perform a fast Fourier transform (FFT) on received signals to recover the OFDM symbols and, hence, the data.
The signals are transmitted from the transmitter to the receiver through a communication channel. In reality, the communication channel may have an effect on the signals when the signals are transmitted. The receiver may need knowledge of the communication channel to remove such effect, in order to accurately recover the data. To facilitate estimation of the communication channel, signals known to both the transmitter and the receiver, i.e., pilot symbols, may be inserted in OFDM symbols at the transmitter, such that the OFDM symbols include both data symbols and pilot symbols. The receiver may perform channel estimation based on resource units, also known as resource blocks, in the received signals, and each of the resource units includes a plurality of OFDM symbols and pilot symbols.
Traditionally, a cellular network may be used to provide wireless communications for a relatively wide area. For example, a cellular network is a radio network including a plurality of radio cells, or cells, each served by a transmitter, also known as a base station. The plurality of cells may cover a relatively wide area compared to the area covered by one cell.
FIG. 1 illustrates a block diagram of a traditional cellular network 100. The cellular network 100 includes a plurality of cells 102-1, 102-2, . . . , and 102-N, served by transmitters 104-1, 104-2, . . . , 104-N, respectively. For example, each of the cells 102-1, 102-2, . . . , and 102-N may have an identification number used to identify the cell. Also for example, the transmitters 104-1, 104-2, . . . , 104-N may transmit signals based on the OFDM technique.
To improve performance of the cellular network 100, each of the transmitters 104-1, 104-2, . . . , 104-N may transmit signals in a plurality of segments/sectors in the cell which the transmitter serves. For example, the cells 102-1, 102-2, . . . , and 102-N may each include a segment 106-1 having a first segment identification number, a second segment 106-2 having a second segment identification number, and a third segment 106-3 having a third segment identification number.
Traditionally, the transmitters 104-1, 104-2, . . . , 104-N may use a same carrier frequency to transmit signals in segments that have a same segment identification number. For example, the transmitters 104-1, 104-2, . . . , 104-N may use a same carrier frequency to transmit signals in the segments 106-1 of the cells 102-1, 102-2, . . . , and 102-N, respectively. As a result, if it is intended that a receiver in the cell 102-1 should receive signals transmitted from the transmitter 104-1, that receiver may also receive signals that have the same carrier frequency and are transmitted from one or more of the transmitters 104-2, . . . , and 104-N, which may cause co-channel-interference (CCI) at the receiver.
To enhance spectrum efficiency, a fractional frequency reuse (FFR) scheme may be used in a cellular network. FIG. 2 illustrates a block diagram of a traditional cellular network 200 based on the FFR scheme. The cellular network 200 includes a plurality of cells 202-1, 202-2, . . . , and 202-N, served by transmitters 204-1, 204-2, . . . , 204-N, respectively. Similar to the cellular network 100 (FIG. 1), each of the cells 202-1, 202-2, . . . , and 202-N may include a first segment 206-1 having a first segment identification number, a second segment 206-2 having a second segment identification number, and a third segment 206-3 having a third segment identification number. In addition, based on the FFR scheme, each segment may further include a plurality of FFR units each having an FFR identification number. For example, for a 2-FFR scheme, each segment may further include two FFR units. Also for example, for a 3-FFR scheme, each segment may further include three FFR units. In FIG. 2, each segment is illustrated as having two FFR units A and A′, B and B′, or C and C′.
The FFR units in the same segment also use the same carrier frequency. As a result, if it is intended that a receiver in the cell 202-1 should receive signals transmitted from the transmitter 204-1, that receiver may also receive signals that have the same carrier frequency and are transmitted from one or more of the transmitters 204-2, . . . , and 204-N, which may cause CCI at the receiver.